The present invention relates to safety circuits, and more particularly, to safety circuits for use with remotely controlled television cameras.
As shown in FIG. 1, it is frequently desirable in certain situations, e.g. sporting, or political events, to have a television camera head (CH) 10 at a considerable distance (sometimes up to 2 miles) from a camera processor and control unit (CPU) 12. An interconnecting transmission line 14 is used in order to both power (280 volts AC is typical) and control the CH from the CPU. Further line 14 provides audio and video channels. One type of line that is used for these purposes has 81 conductors. This is expensive, inconvenient to handle, and thick. In the RCA TK-47 camera, time-division multiplexing is used in order to reduce the required number of conductors to 30. This reduces cost and bulk as compared to the 81 conductor cable. To maximize safety, both the 30- and 81-conductor cables use screw-on connectors, with the "hot" side (the side of the cable having the power) having a female connector. As a further safety measure in the 30 conductor cable, one conductor, called a "sense wire" is connected to ground at CH 10. If the cable is broken or disconnected, a "high" logic voltage level is applied to this wire, which high logic level actuates a logic circuit to trip a relay in CPU 12. The tripped relay causes power to be no longer applied to the cable 14.
Recently, to still further reduce the bulk and cost of the cable, triaxial ("Triax") cable 15, as shown in FIG. 3, has been used. This cable 15 comprises an inner conductor 16, insulation layer 18, inner shield 20, insulation layer 22, outer shield 24, and finally an outer insulation layer 26. The outer shield 24 is connected to the casing of the CPU 12 and CH 10. The AC power and signals are all transmitted using inner shield 20 and inner conductor 16 using triax interface circuits 28 and 30 shown in FIG. 2. Circuits 28 and 30 are shown in more detail in FIG. 4. In this arrangement, signals at CPU 12 are frequency division multiplexed by filters (not shown) and applied to an RF (radio frequency) modulator 32. The resulting modulated signal is applied to LPF (low pass filter) 34, and the resulting filtered signal is applied to capacitor C1. Capacitor C1 has a high voltage rating and a low reactance for RF signals, and therefore serves to block the AC power from entering the RF circuits as explained below. Thus the filtered signal is applied to inner conductor 16. Also applied to conductor 16, as well as inner shield 20 is AC power from a source (not shown) through isolation inductors L1 and L2. These inductors block the RF signals from the AC power source while letting AC power pass through them. Triax 15 conveys the AC and RF signals to CH 10 where they are applied to blocking capacitor C3 and isolation inductors L3 and L4. It is noted that isolation capacitors C2 and C4 are coupled to inner shield 12 and the RF source ground at CPU 12 and CH 10 respectively. At CH 10, the RF signals pass through blocking capacitor C3 and are applied to LPF 36 and HPF (high pass filter) 38. However, since the RF signals were passed through LPF 34, they will only pass through LPF 36 and not HPF 38. From LPF 36 the RF signals are applied to a demodulator (not shown) and then baseband filters (not shown) to separate them. Further, the RF signals cannot pass through isolation inductors L3 and L4. The AC power cannot pass through capacitor C3, but it does pass through inductors L3 and L4 to power the rest of CH 10 (not shown).
At CH 10 RF signals from a high frequency modulator (not shown) are applied to HPF 38 and then pass through capacitor C3. The RF signals cannot pass through LPF 36 or inductors L3 and L4, and thus substantially all of the RF signals are applied between inner conductor 16 and inner shield 20. Triax 15 conveys the RF signals to CPU 12 where they pass through capacitor C1 and then HPF 40 to synchronous demodulator 42. The high frequency RF signals from CH 10 cannot pass through inductors L1 and L2 or LPF 34. From demodulator 42 the now demodulated signals are applied to baseband filters (not shown) for their separation.
It will be seen that no sense wire (as explained above) is present in the triax system of FIG. 4.
It is therefore desirable to improve the safety of a triaxial cable system.